Leaf Hydraulics and Biomechanical Strength of two Evergreen Species of Chaparral Shrubs in the Family Rhamnaceae

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Southern California chaparral shrubs are among the most drought tolerant plants on earth. The Mediterranean-type climate of California is characterized by cool, wet winters and hot, dry summers. Negative pressures generated by water stress within the xylem of these plants during the yearly dry season can cause cavitation and embolism of water conduits, rendering them inactive. Previous research suggests that resistance to this process in stems is correlated with mechanical strength and inversely correlated with hydraulic conductivity. This study tests whether the same tradeoff exists within the leaf tissues of two evergreen species of chaparral shrubs within the family Rhamnaceae that differ in their resistance to water stress. Mechanical tests, pressure volume curves, and the evaporative flux method to estimate leaf hydraulic conductance were used to characterize the mechanical and hydraulic properties of leaves from Rhamnus ilicifolia and Frangula californica. Parameters measured included stress at break (N/mm2), young’s modulus, leaf mass per area (LMA), water potential at turgor loss point (Ψtlp), relative water content at turgor loss point (RWCtlp), bulk modulus of elasticity (ε), and leaf hydraulic conductance (kleaf). Compared to F. californica, R. ilicifolia leaves are mechanically stronger, more drought tolerant, and displayed significantly higher hydraulic conductivity at midday water potentials. Therefore, the tradeoff observed between water transport efficiency, transport safety, and mechanical strength in stems does not occur in leaf tissues of the two species examined. These results are likely due, in part, to the complex pathway that water takes through the leaf during transpiration. Further research is needed to determine how anatomical differences between leaves and stems affect their respective water relations.